This invention relates to the modulation of binary data using .pi./4-DQPSK (Differential Quadrature Phase Shift Keying). In particular, this invention provides a real time encode/decode (Codec) operation which acts on message binary data to ensure that particular phase states can be achieved which are consistent with the "Cellular System Dual-Mode, Mobile Station-Base Station Compatibility Standard" IS-54 or any other similar standard.